Bottlenose dolphins (Tursiops truncatus) in North Dalmatia, Croatia: Occurrence and demographic parameters

The common bottlenose dolphin (Tursiops truncatus) population along the eastern Adriatic coast is believed to comprise discrete communities, yet many of them are unstudied. This study provides the first description of occurrence and demographic parameters for the community inhabiting waters of North Dalmatia. Dedicated boat-based surveys conducted in summer months from 2013 to 2017 resulted in 13,896 km of research effort and 284 dolphin sightings from which 336 individuals were identified. Site-fidelity analysis revealed that 52.1% of encountered marked adults occur in the area regularly or occasionally. No significant difference among years could be found for mean group sizes (5.73-7.46) and group encounter rates (1.159-1.942 groups/100 km). Pollock's Robust Design models estimated annually variable adult apparent survival (0.737-0.986) and constant temporary emigration rate of 0.172. The estimated abundance varied annually from 116 to 138 individuals and showed a negative slope, but a significant trend could not be confirmed. Age-dependent models estimated first-year calf survival at 0.875. These results provide a baseline for informed management of two Sites of Community Importance, and a benchmark for future monitoring in North Dalmatian waters, an area under significant anthropogenic pressure.

Automated summary

This study provides the first description of occurrence and demographic parameters for the community of common bottlenose dolphins inhabiting waters of North Dalmatia. The estimated abundance varied annually with no significant trend confirmed. Site-fidelity analysis revealed that 52.1% of encountered marked adults occur in the area regularly or occasionally.